OK, OP here again.
I've decided to to call the framework "Scaffold" for hopefully obvious reasons. I went ahead and created a shithub for it at https://github.com/SpaceBudokan/Scaffold so we can at least pretend to make this a reality one day.
The basic premise is that Scaffold is a collection of API's to assemble a browser from various parts. >>830622 has a very good idea of what I'm talking about. Scaffold is the arrows in his pic. We create APIs so that we can modularly swap out any basic part of the browser. One of my main questions is how far to take it. I was orginally thinking of doing all the interface work in GTK+, but even the interface could be put behind an API, so that one could define general widgets, and they can be used with GTK+, Qt, fltk, translated to text for use in the terminal, whatever.
Also an API for talking with and between the layout and javascript engines. I feel like I've already said enough about this part, but it's what I think makes it truly unique.
Lastly would be an API to make more conventional plugins. Still, it would allow one to make extremely customizable things like some kind of exotic address bar like Vermette talked bout, weird tab systems, or even some other weird way or organizing your pages. Maybe for whatever random reason you want you tabs, address bar, and and each in a separate window from where your webpage is displayed. You could do that.